Harvard University Medical School
How Do You Know You Know?
Origin of human consciousness may lie in newly pinpointed
brain network
Philosophers
have long struggled to define human consciousness. Now, a team of researchers
led by neurologists at Harvard Medical School and Beth Israel Deaconess Medical
Center has pinpointed the regions of the brain that may play a role maintaining
it.
Their
findings, which have already garnered multiple awards from the American Academy
of Neurology, were published Nov. 4 in that society’s journal, Neurology.
“For the
first time, we have found a connection between the brainstem region involved in
arousal and regions involved in awareness, two prerequisites for
consciousness,” said Michael Fox, HMS assistant professor of neurology at Beth
Israel Deaconess. “A lot of pieces of evidence all came together to point to
this network playing a role in human consciousness.”
Classical
neurology holds that arousal and awareness are two critical components of
consciousness. Arousal is likely regulated by the brainstem—the portion of the
brain, contiguous with the spinal cord, that is responsible for the sleep-wake
cycle and cardiac and respiratory rates.
Awareness,
another critical component of consciousness, has long been thought to reside
somewhere in the cortex, the outer layer of the brain responsible for many of
its higher functions.
The
researchers analyzed 36 patients with brainstem lesions; 12 led to coma and 24
did not.
Mapping
the injuries revealed that a small coma-specific area of the brainstem—the
rostral dorsolateral pontine tegmentum—was significantly associated with coma.
Ten out of the 12 coma-inducing brainstem lesions involved this area, while
just one of the 24 control lesions did.
Armed
with that information, Fox and colleagues, including lead author David B.
Fischer, HMS clinical fellow in medicine at Brigham and Women’s Hospital, used
a wiring diagram of the healthy human brain, based on a large, shared data set
called the Human Connectome, to identify which other parts of the brain were
connected to these coma-causing lesions.
Their
analysis revealed two areas in the cortex of the brain that were significantly
connected to the coma-specific region of the brainstem. One sat in the left,
ventral, anterior insula, the other in the pregenual anterior cingulate cortex
(pACC). Both regions have been implicated previously in arousal and
awareness.
“We now
have a great map of how the brain is wired up in the Human Connectome,” said
Fox, who is also director of the Laboratory for Brain Network Imaging and
Modulation and the associate director of the Berenson-Allen Center for
Noninvasive Brain Stimulation at Beth Israel Deaconess.
“We can
look at not just the location of lesions, but also their connectivity," he
said. "Over the past year, researchers in my lab have used this approach
to understand visual and auditory hallucinations, impaired speech and movement
disorders. A collaborative team of neuroscientists and physicians had the
insight and unique expertise needed to apply this approach to consciousness.
The team
included co-lead author, Aaron Boes, and co-senior author, Joel Geerling, both
formerly of Beth Israel Deaconess and now of University of Iowa Carver College
of Medicine.
Finally,
the team investigated whether this brainstem-cortex network was functioning in
another subset of patients with disorders of consciousness, including coma.
Using a special type of MRI scan, the scientists found that their newly
identified “consciousness network” was disrupted in patients with impaired
consciousness.
The
findings—bolstered by data from rodent studies—suggest the network between the
brainstem and these two cortical regions plays a role in maintaining human
consciousness.
“The
added value of thinking about coma as a network disorder is it presents
possible targets for therapy, such as using brain stimulation to augment
recovery,” Boes said.
A next
step, Fox notes, may be to investigate other data sets in which patients lost
consciousness to find out if the same, different or overlapping neural networks
are involved.
“This is
most relevant if we can use these networks as a target for brain stimulation
for people with disorders of consciousness,” said Fox. “If we zero in on the
regions and network involved, can we someday wake someone up who is in a
persistent vegetative state? That’s the ultimate question.”
This
work was supported by the Howard Hughes Medical Institute, the Parkinson’s
Disease Foundation, the NIH (Shared Instrument Grants S10RR023043, K23NS083741,
R01HD069776, R01NS073601, R01NS085477, R21MH099196, R21NS082870, R21NS085491,
R21HD07616, R25NS065743, R25NS070682, T32 HL007901 and P01HL095491), American
Academy of Neurology/American Brain Foundation, Sidney R. Baer, Jr. Foundation,
Harvard Catalyst | The Harvard Clinical and Translational Science Center, the
Belgian National Fund for Scientific Research, the European Commission, the
James S. McDonnell Foundation, the European Space Agency, Mind Science
Foundation, the French Speaking Community Concerted Research Action
(ARC-06/11-340), the Public Utility Foundation “Université Européenne du
Travail,” Fondazione Europea di Ricerca Biomedica, the University and
University Hospital of Liège, the Center for Integrative Neuroscience, and the
Max Planck Society.
Adapted
from a Beth Israel Deaconess news release.
Source: https://hms.harvard.edu
